1. Technical Field of the Invention
The present invention relates generally to an improved structure of a combustion pressure sensor for measuring the combustion pressure of an internal combustion engine, and more particularly to such a combustion pressure sensor designed to ensure the stability of output characteristics and sensitivity thereof.
2. Background Art
FIG. 19 shows one of typical combustion pressure sensors, as taught in Japanese Patent First Publication No. 2002-327919.
The combustion pressure sensor consists of a hollow cylindrical housing 10, a load detector 12, and a center shaft 111. The housing 10 is installed in a cylinder head of an internal combustion engine. The load detector 12 is sensitive to a physical pressure load to produce an electric signal indicative thereof. The center shaft 111 is disposed within the housing 10 and works to transmit the combustion pressure in the combustion chamber of the engine to the load detector 12.
The load detector 12 includes a laminate of piezoelectric elements 122 and electrodes 129. The laminate is pressed by a nut 119 screwed on the center shaft 111 to apply a preload to the piezoelectric elements 122. When the compressive pressure acting on the piezoelectric elements 122 changes with a change in pressure within the combustion chamber of the engine, it will cause the piezoelectric elements 122 to produce an electric signal in the form of voltage as a function of the change in the compressive pressure.
Improving the efficiency of transmitting the load to the piezoelectric elements 122 is effective in decreasing a unit-to-unit variation in sensitivity of combustion pressure sensors or stabilizing output characteristics of each combustion pressure sensor. To this end, it is required for combustion pressure sensors of the above type to ensure complete surface-to-surface contacts between the piezoelectric elements 122 and the electrodes 129. Particularly, it is important to achieve complete contacts between the entire surfaces (will also be referred to as pressure-applied surfaces below) of the piezoelectric elements 122 and surfaces of parts nipping the piezoelectric elements 122 therebetween. This requires application of a great preload to the piezoelectric elements 122 and the nipping parts. The inventors of this application performed tests on the above type of combustion pressure sensors and found that a preload of approximately 3000N is required for decreasing the unit-to-unit variation in sensitivity of the combustion pressure sensors or stabilizing output characteristics of the combustion pressure sensors.
The application of a preload of as much as 3000N, however, results in breakage of the center shaft 111, etc. or physical damage to joints in the sensor.
The surface-to-surface contacts between the piezoelectric elements 122 and the electrodes 129 may alternatively be achieved by increasing the flatness or parallelism of the surfaces of the piezoelectric elements 122 and the electrodes 129. This requires a high degree of machining accuracy, thus resulting in increased production costs of the combustion pressure sensors.